IL-4 is a pleiotropic immune regulatory cytokine and is being tested in the clinic for the treatment of a variety of human cancers. The soluble form of IL-4 receptor (IL-4R) is being used for the treatment of bronchial asthma. Since we have discovered that a variety of solid human cancer cells express IL-4R, studies are being undertaken to characterize structure, function, signal transduction and targeting of IL-4R expressed on immune cells and cancer cells. IL-4R directed targeting of a Pseudomonas exotoxin (PE), and regulation and interaction of IL-4R with other cytokines, cytokine receptors and regulatory proteins such as HIV-1 tat is also being investigated. These ongoing studies will reveal the mechanism of IL-4 action and IL-4 induced toxicity. A. Structure and function of IL-4R on tumor cells. Crosslinking studies have demonstrated that IL-4R on human cancer cells are composed of two major proteins of 140 kDa and 70 kDa. Immune cells also expressed two to three IL-4 binding proteins of 140 kDa, 70 kDa and 64 kDa depending on cell types examined. It has been shown that IL-2R gamma chain is associated with IL-4R and is a necessary component for IL-4 signalling. The 64 kDa protein appears to be gamma chain, however, the identity of the 70 kDa protein is not clear. Immunoprecipitation studies demonstrated that gamma is not a component of IL-4R in solid cancer cells. We have demonstrated that solid cancer cells express large numbers of IL-13R. Since IL-13 competed for the binding sites of IL-4 and the size of IL-13R binding protein is similar to that of IL-4R 70 kDa protein we proposed that IL-13R is a component of IL-4R. B. IL-4R directed targeting of a Pseudomonas exotoxin. The IL-4R expressed on human tumor cells were targeted with a chimeric protein comprised of IL-4 and PE. Newer generations of IL4-PE toxins have improved binding. In addition, this chimeric protein was more cytotoxic to IL-4R positive tumor cells. Anti-tumor studies were carried out in nude mice using new toxin in xenograft model bearing human epidermoid carcinoma. Circularly permuted IL4-toxin can cause durable complete responses in mice. Pre-clinical studies are underway to explore optimal route and schedule of therapy and type of human tumor which will be most susceptible. Since many chimeric proteins composed of cytokines, antibody and PE are being used in the clinic these studies will provide insight into the mechanism of action and toxicity. C. IL-4 induced signal Transduction: IL-4 has been shown to induce phosphorylation of insulin response substrate (IRS-1), IL-4R and Janus kinases (JAK) 1, and 3. The gamma chain of IL-4R has been shown to be required for the phosphorylation of IRS-1 and JAK3 in IL-4 signalling pathway. Our studies show that in the absence of gamma chain, IL-4 can induce phosphorylation of IRS-1 and JAK3 is not involved in IL-4R signalling pathway in tumor cells. Studies are ongoing to identify IL-4R signalling differences between immune cells and cancer cells.